The present invention relates to acoustic logging tools, and in particular to aspects of a receiver section for an acoustic logging tool in which the interfering effect of flexural waves is minimised.
Acoustic logging tools are used in the evaluation of formations surrounding boreholes such as are used for extraction of hydrocarbons. FIG. 1 shows a schematic view of a prior art acoustic logging tool such as the DSI Dipole Sonic Shear Imager of Schlumberger. The tool comprises a sonde 10 which is lowered into a borehole 12 by means of a wireline cable 14. The cable is used both to support the sonde 10 and provide a power, control signal and data transmission path to the surface unit 16. The sonde 10 includes a transmitter section TX capable of generating dipole and monopole acoustic signals, a sonic isolation joint SIJ, a receiver section RX, and an electronics cartridge EC. The receiver section RX includes a number of spaced receiver stations, typically eight stations are used, and each station has typically four piezoelectric sensors for measuring pressure in the borehole due to passing acoustic waves. Examples of various aspects of such a tool can be found in U.S. Pat. No. 4,850,450; U.S. Pat. No. 4,862,991; U.S. Pat. No. 4,872,526; U.S. Pat. No. 5,036,945 and U.S. Pat. No. 5,043,952.
In dipole logging, the transmitter TX generates a dipole acoustic signal which propagates along a number of possible paths to the receivers RX1, RX2 (only two station are shown here instead of the usual eight for purposes of clarity). These paths which are shown schematically in FIG. 2, are (1) along the sonde itself, (2) through the fluid filling the borehole, and (3) as a formation/borehole mode in which the signal passes from the transmitter through the fluid in the borehole to the formations surrounding the borehole where a surface wave mode of the borehole is set up (this is a dispersive mode whose slowness dispersion characteristics are determined both by the properties of the formation surrounding the borehole, the dimensions of the borehole and the properties of the borehole fluid), then back into the borehole fluid and then to the receivers RX1, RX2. Since the purpose of acoustic logging is to determine the properties of the formation surrounding the borehole, it is the last path which is of interest, the signals passing along paths (1) and (2) giving no information about the formations and so interfering with the evaluation.
The speed (or xe2x80x9cslownessxe2x80x9d) of propagation of the acoustic signal is dependent on the physical nature of the medium through which it propagates; typically the stiffer the medium, the faster the propagation. The slowness of the pressure field signal through the borehole fluid is typically around 200 xcexcs/ft. The slowness of the tool flexural wave depends on the particular design of the tool but will typically be  greater than 700 xcexcs/ft. The slowness of the formation/borehole flexural wave signal (the signal of interest) can range from about 100 xcexcs/ft to 1000 xcexc/ft in typical formations logged by these tools. The presence of the sonic isolation joint SIJ between the transmitter TX and the receiver RX goes some way to reducing the signal propagated along the sonde body (the xe2x80x9ctool signalxe2x80x9d), an example of which is described in U.S. Pat. No. 4,862,991. However, this in itself is not enough, especially when dealing with the propagation of flexural waves along the tool. One approach is to provide a housing for the sonde which is configured to delay the tool signal sufficiently that it does not interfere with the formation signal. An example of this is found in U.S. Pat. No. 4,850,450 and in the Schlumberger DSI tool. The receiver section of the DSI tool includes a central mandrel around which are mounted alternate, Teflon hydrophone mounts and steel spacers connected together to form a continuous structure. The hydrophones are aligned radially (polarisation of the piezoelectric stack is aligned with the radius of the tool). The slotted sleeve has alternate window and slotted structures. The window section has 10 bars defining the windows (each window defining a 20xc2x0 arc) and four rows of regular circumferential slots (each slot defining a 70xc2x0 arc). The slotted sleeve is shown in FIGS. 3a and 3b. 
Another approach is to avoid using a rigid continuous housing in the receiver section. U.S. Pat. No. 5,289,433, U.S. Pat. No. 5,343,001 and U.S. Pat. No. 5,731,550 describe acoustic tools in which the receiver includes receiver stations separated by connectors or spacers which include some compliant or acoustically isolating material at the contacting surfaces. The approach described in the ""433 and 001 patents is that the receiver section lacks sufficient strength or rigidity to be used in tough logging conditions or non-vertical wells. In such conditions, a sleeve must be used and the problem of tool signal interference is found. The connectors in the ""550 patent are configured to allow greater rigidity in compression but retain an element of sonic isolation in tension which is the normal logging condition.
While slotted sleeve tools do have good mechanical properties, certain problem can be encountered in slow formations, when sleeve arrivals in the interfere with the slow formation arrivals, and when inconsistencies arise in the waveforms from the receiver section due to tool vibration excited by borehole waves. It is an object of this invention to attempt to address such problems.
The present invention adopts certain principles to provide a structure which has a flexural dispersion (slowness vs. frequency) which does not overlap with the flexural dispersion of the borehole in the formations of interest, and in which the receiver section is constructed in a manner to optimise detection of the signal of interest while minimising the interfering signal and the sensitivity of the receiver section to coupling with the borehole mode of vibration.
In accordance with one aspect of the invention, a sleeve for surrounding the receiver section of an acoustic logging tool, at least in the region of the receiver stations, has alternating first and second apertured portions spaced along its length, wherein
(a) the first apertured portion has elongate axial bar elements separated by windows in a circumferential arrangement, the windows being wider than the bars. and
(b) the second apertured portion has rows of circumferentially elongate slots, the slots having a relatively narrow centre portion and relatively enlarged end portions.
In a second aspect of the invention, a receiver section for an acoustic logging tool comprises a number of receiver stations spaced along a tool body, each station including a number of polarised pressure sensors spaced around the circumference of the tool body, the axis of polarisation of the sensors being parallel to the axis of the tool body.
The first apertured portion of the sleeve (the window section) has a reduced number of bars of increased length and larger windows compared to a standard sleeve. This tends to reduce the spring constant/increase flexibility of this portion so increasing the flexural slowness of the sleeve (speed of flexural propagation along the sleeve). In one embodiment, two alternating window widths are chosen, for example alternating 45xc2x0 and 25xc2x0 windows. It is preferred to configure the window section so as to inhibit coupling with higher modes of vibration (such as hexapole). This is achieved by selecting the number of windows and the relative dimensions of the windows (for example, alternating sizes as described above).
The second apertured portion (the slotted section) portions are provided with typically three rows of thin circumferential slots with enlarged portions at the ends (xe2x80x9cdumb-bellxe2x80x9d shaped slots). The axial length of the slotted section can be reduced while the mass is essentially the same as the corresponding structure in the prior art sleeve with regular slots. The ratio of the width of the centre portion of the slots to the radius of the end portions is typically at least 1:4, 1:6 being preferred. The slots typically define 70xc2x0 arcs. Each row of slots is displaced relative to the adjacent row(s). This displacement is conveniently 90xc2x0 although other angles might be appropriate.
In the receiver section, the pressure sensor (hydrophone) mounts are preferably made massive, constructed from steel. The hydrophones themselves are mounted axially (vertically) so as to be less susceptible to tool vibration caused by coupling of borehole modes. The receiver section has a central mandrel to which spacers are attached, each spacer carrying the weight of the receiver mount above through a compliant pad. Thus each receiver mount is essentially independent of its neighbours.
A basic concept which is used in constructing a tool embodying the present invention is to ensure that the flexural dispersion (slowness vs. frequency) of the tool does not overlap with the flexural dispersion of the formations of interest at the frequencies of interest. For example, where formations having a slowness of 1200 xcexcs/ft are to be measured, the tool is designed such that tool flexural arrivals do not occur below that speed.